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Topic 3 Math/Drugs Calculations

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IV Giving Sets Generally there are 2 types of giving sets in use and they deliver drops of different sizes Blood giving sets 15 or 20 drips/ml. these are known as macrodrips The more commonly used metrisets deliver 60/drips/ml. These are known as microdrips.

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IV calculation For drops per minute the rule is: Rate (drops/minute) = Volume (in ml) x Drip factor Time (minutes)

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Example 1 A 1L bag of N/saline is to run over 6 hours. You are using a giving set which delivers 20 drops/ml. What should the flow rate be set at? Rate = 1000 x 20 convert the 1L to 1000ml 6 x 60 1 = 1000 x 20 common factor 10 & 4 360 1 = 2000 36 = 500 Long division 9 = 55.55 = 56 drops/ minute Rounded up to nearest whole number

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Example 2 A 500ml bag of Hartman’s solution is to run over 4 hours. The giving set delivers 20 drops/ml. What should the flow rate be?

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Solution – Example 2 Rate = 500 x 20 4 x 60 1 = 500 x 20 240 1 = 500 12 = 125 3 = 42 drops/minute

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Example 3 A patient is to receive 500ml of 5% Dextrose over 2 hours using a giving set which delivers 20 drops/ml. Calculate the drip rate in drops/minute.

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Solution - Example 3 Rate = 500 x 20 2 x 60 = 10000 120 = 250 3 = 83 drops/minute

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Example 4 A patient has to have an IV set up to deliver 1L of fluid over 4 hours, using a metriset (60 drops/ml). What should the flow rate be?

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Solution - Example 4 Rate = 1000 x 60 4 x 60 = 60000 240 = 2000 8 = 250 drops/minute

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Example 5 A dehydrated adult is ordered 1L of normal saline over 2 hours, using a macrodrip (20 drops/ml) IV giving set. Calculate the drip rate in drops/minute.

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Solution - Example 5 Rate = 1000 x 20 2 x 60 = 20000 120 = 1000 6 = 167 drops/ minute

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IV calculations You made need to calculate how long an IV infusion will take to run through, if you know the drip rate re-organise the previous rule For the time for a drip to run the rule is: Time (in minutes) = Volume (in ml) x Drip factor Rate (drops/min)

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Example 1 There is 500ml left to run through the IV metriset. The drip rate has been set at 50 drops/minute. How long will it take to run through? Time (in minutes) = 500 x 60 50 1 = 10 x 60 1 1 = 600 minutes = 10 hours

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Example 2 There is 700ml of N/saline left to run through the IV metriset. The drip rate is set at 40 drops/minute. How long will it take to complete?

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Solution - Example 2 Time (in minutes) = 700 x 60 40 1 = 700 x 6 4 1 = 700 x 3 2 1 = 2100 2 = 1050 =17 ½ hours = 17 hours 30 minutes

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Example 3 How long will an IV take to run through if the drip rate is set at 80 drops/minute and there are 300ml left using a drip factor of 20 drops/ml?

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Solution - Example 3 Time (in minutes) = 300 x 20 80 = 6000 80 = 600 8 = 75 minutes OR = 1 ¼ hr

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Example 4 How long will 300ml of IV packed cells take to run through at 30 drops/minute using a blood giving set (15 drops/ml)?

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Solution - Example 4 Time (in minutes) = 300 x 15 30 = 4500 30 = 450 3 = 150 minutes OR = 2 ½ hr

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Example 5 How long will 2l of Normal Saline take if it is running at 100ml/hour using a metriset?

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Solution - Example 5 Time (in minutes) = 2000 x 60 100 = 120000 100 = 1200 minutes OR = 20 hours

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IV Calculations Sometimes you made need to calculate the hourly volume of an IV. To do this reorganise the previous rule For volume per hour the rule is Volume per hour = Total Volume Time (hours)

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Example 1 The doctor has ordered 1500 ml of fluid to be given over 5 hours. What should the hourly volume be in ml per hour? Volume = 1500 5 = 300 ml/hr

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Example 2 The client is ordered 1.2L of Hartmann’s solution over the next 6 hours. What should be the hourly volume in ml/hour?

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Solution - Example 2 Volume = 1200 6 = 200 ml/hr

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Example 3 An IV containing 600ml has to run through in the next 4 hours. What volume needs to run through every hour?

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Solution - Example 3 Volume = 600 4 = 150 ml/hr

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Example 4 A client’s IV is commenced at 1000HR. It contains 1L and is to run through by 2000HR. What should the hourly volume be (in ml per hour)?

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Solution - Example 4 Volume = 1000 10 = 100 ml/hr

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Example 5 What volume is needed every hour if a client must have 700 ml in 10 hours?

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Solution - Example 5 Volume = 700 10 = 70 ml/hr

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Paediatric Drug Doses Clarke’s Body Weight Rule (used for children aged 1 and over) Child’s dose = Weight of child (kg) x Adult dose Average adult weight (70kg)

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Example 1 A child weighing 7kg is to be given ampicillin. The adult dose is 500mg. How much should the child be given? Child’s dose = 7 x 500 70 = 1 x 500 10 = 50 mg

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Example 2 A child weighing 10 kg is to have penicillin and the adult dose is 500mg. How much would you give?

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Solution - Example 2 Child’s dose = 10 x 500 70 = 1 x 500 7 = 500 7 = 71.4 = 71 mg

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Paediatric Drug Doses Clark’s Body Surface Area (BSA) Rule Child’s dose = Surface area of child (m²) x Adult dose average surface area of adult (1.7 m²)

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Example 1 A child whose BSA is 0.7 m², is ordered penicillin. The adult dose is 1g. Calculate how much to give. Child’s dose = 0.7 x 1000 1.7 = 7 x 1000 17 = 7000 17 = 412 mg

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Example 2 A child whose BSA is 0.6 is ordered pethidine. The adult dose is 100 mg. How much would you give?

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Solution - Example 2 Child’s dose = 0.6 x 100 1.7 = 6 x 100 17 = 600 17 = 35 mg

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Paediatric Drug Doses Freid’s Rule (used for infants under 1yr old) Child’s dose = Age in months x Adult dose 150

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Example 1 A six month old child is to be given amoxycillin with clavulanic acid (Augmentin). The adult dose is 500mg. How much should the child be given? Child’s dose = 6 x 500 150 = 6 x 10 3 = 60 3 = 20 mg

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Example 2 A 4 month old is to be given tetracycline. The adult dose is 250mg. How much would you give?

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Solution - Example 2 Child’s dose = 4 x 250 150 = 4 x 5 3 = 20 3 = 6.6666 = 7 mg

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Paediatric Drug Doses Young’s Rule (used for children aged 2 to 12) Child’s dose = Age in years x Adult dose age + 12

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Example 1 The adult dose for metronidazole (Flagyl) is 500mg. How many milligrams should you give a 10 year old? Child’s dose = 10 x 500 10 + 12 = 10 x 500 22 = 5000 22 = 227 6/22 mg = 227 mg

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Example 2 An 8 year old is ordered ampicillin. The adult dose is 500mg. How much should you give the client?

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Solution - Example 2 Child’s dose = 8 x 500 8 + 12 = 8 x 500 20 = 8 x 50 2 = 4 x 50 = 200mg

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